Replace deprecated HexStringToBytes API

Deprecated in http://google3/third_party/absl/strings/escaping.h;l=234;rcl=677488070

#codehealth

PiperOrigin-RevId: 683197220

quiche/http2/test_tools/http2_random.cc

diff --git a/quiche/http2/test_tools/http2_random.cc b/quiche/http2/test_tools/http2_random.cc
index bfd4872..1e15335 100644
--- a/quiche/http2/test_tools/http2_random.cc
+++ b/quiche/http2/test_tools/http2_random.cc
@@ -19,7 +19,8 @@
 }
 
 Http2Random::Http2Random(absl::string_view key) {
-  std::string decoded_key = absl::HexStringToBytes(key);
+  std::string decoded_key;
+  QUICHE_CHECK(absl::HexStringToBytes(key, &decoded_key));
   QUICHE_CHECK_EQ(sizeof(key_), decoded_key.size());
   memcpy(key_, decoded_key.data(), sizeof(key_));
 }